It finally works. Turned out to be a problem with the voltage regulator as it is rated only for 100mA max it couldn't deliver enough power for the drv reference and power the attiny. Not sure why it was causing this kind of problem, but after exchanging the old regulator (MCP1792T-3302H/CB) for a new one (AP2210N-3.3TRE1, rated for 300mA), I just seemed to have to laying around, everything works just fine. No overheating, no other problems. Thanks for your help, I really appreciate it.